1. Field of the Invention
The present invention relates to an alignment apparatus for accurately aligning a plurality of medical images and to a program therefor.
2. Description of the Related Art
Using the fact that attenuation of a radiation having passed through a subject becomes different depending on a material comprising the subject, energy subtraction processing has been known wherein two component images comprising different signal components are separated from two radiation images obtained by irradiation of high and low energy radiations on a subject. According to such energy subtraction processing, soft tissue images can be generated by removing bones from chest radiation images, and shadows in soft tissues can be observed without being blocked by the bones, for example.
In the case where radiation images of high and low energy used in such energy subtraction processing are radiographed by two-shot method, a high-energy radiation and a low-energy radiation are irradiated on a subject at an interval of several hundred milliseconds. However, if the subject moves during the radiography, a position thereof becomes different between the images. Therefore, a component image generated by subtraction processing includes an artifact.
In order to reduce artifacts caused by alignment errors in energy subtraction processing, an alignment method carrying out non-linear transformation on either one of images before subtraction processing has been proposed. More specifically, registration by template matching is used. In registration using template matching, a local region of either one of images is cut as a template, and a region corresponding to the template is searched for in the other one of the images, to find how each pixel has moved (that is, a shift vector). Based on the shift vector, warping is carried out for registration. By finding the shift vectors for all pixels in one image, the pixels can be related to all pixels in the other image. However, in consideration of computational efficiency and alignment accuracy, control points are generally disposed to form a grid-like pattern, and shift vectors are found at the control points. Pixels between the control points are related to each other in the two images by interpolation of the surrounding shift vectors, and the images are aligned by warping according to the correspondence between the pixels.
In template matching, a correlation value between two images is used to find a region that corresponds to a template. However, finding only a similar region may not represent accurate correspondence, leading to mal-registration. Therefore, in order to solve this problem of mal-registration, a method has been proposed in U.S. Pat. No. 6,661,873, for example. In this method, a preliminary soft-tissue template is generated from two-shot images by using subtraction processing, and edge intensity therein is used as a criterion for alignment.
Alternatively, another method to reduce mal-registration has been proposed in Japanese Unexamined Patent Publication No. 2000-342558. In this method, tissues are divided in advance into regions in alignment processing before subtraction processing. Degrees of similarity between the regions are found to find a degree of similarity between a template and a radiation image, and a result of weighted addition of the degrees is then used as a criterion for alignment.
In the template matching described above, a size of a template to be cut, a similarity (an alignment criterion) to the template, a region of template matching, and the accuracy of template matching affect the final accuracy of alignment and computational cost. Therefore, optimal parameter setting is required.
However, in the case of two-shot radiography, an organ in motion such as the heart deforms more greatly than other regions, and accurate alignment is not realized even in the case of applying the same criteria to entire images. For example, in two-shot energy subtraction processing using a flat panel detector (FPD), the interval of two-shot radiography is approximately 200 to 500 milliseconds. However, the heart rate at rest is approximately 60 to 70 for men and 65 to 75 for women, meaning that one heartbeat takes about 800 milliseconds to 1 second. Therefore, since the heart is always in motion, a region around the heart changes substantially.